Info

Graduated symbols

Graduated symbols are the best way to visualize absolute numbers like population size. Usually the area of the symbols is made proportional with the number to be displayed. This is especially important if the values are far apart.

Methods

QGIS 1.8 offers four methods to display graduated symbols. All can be found in the dialogue ‘Layer properties’, on various tabs. The next table summarizes advantages and disadvantages of these four methods. ‘Old symbology’ is not discussed here.

Method

Advantages

Disadvantages

1 Style / Advanced / Size scale field

Overlapping symbols remain visible

Transparancy

Only for point layers

Only linear scaling

Large values troublesome

2 Overlay / Pie chart

For point and polygon layers

Non-linear scaling

Overlapping symbols difficult to discern

No transparancy

3 Diagrams / Pie chart

For point and polygon layers

Only linear scaling

Large circles not displayed

4 Overlay / Proportional SVG

For point and polygon layers

Only linear scaling

Larger circles get broad borders

None of these methods is perfect. This article only describes methods 1 and 2 in more detail.

Method 1 Style / Advanced / Size scale field

This method only works with point layers. In this example a text file is used with data on the Dutch provinces. A shapefile with province borders is used as background (download sample data).

First open the shapefile with province borders. Then add the data with ‘Layer / Add delimited text layer’. The attribute table is shown on the next image.

Attribute table

Open the ‘Layer properties’ of this layer.

Settings for graduated symbols

Use the ‘Advanced’ button to select the field to be displayed. In this case ‘bev2011w’ (square root of population size). Change the units into ‘Map units’. Change ‘Size’ to set circle size and ‘Color’ to set circle colour. The result can be checked by clicking the ‘Apply’ button. The result is shown on the next image.

Map with graduated symbols 1

Method 2 Overlay / Pie chart

This method works with both point and polygon layers. Select the layer containing the data, open the ‘Layer properties’ dialogue and go to the ‘Overlay’ tab. If this tab is missing, first activate the plugin ‘Display diagrams’. Tick the option ‘Display diagrams’ and select ‘Pie charts’ as diagram type. Add the attribute ‘bev2011′. Double click on the field to set the color. As ‘Classification attribute’ the same field should be selected. This field determines circle size.

Now click on ‘Find maximum value’ and enter the maximum circle size (‘Size’). The result can be tested by pressing the ‘Apply’ butten. Currently, only one classification type is available. The name is misleading: symbols are not scaled linearly at all. The result would have been very different from the result of the first method.